MACHAERODUS, or Machairodus, the typical genus of a group of long-tusked extinct cats, commonly known as sabre-tooths. Although best regarded as a sub-family (Machaerodontinae) of the Felidae, they are sometimes referred to a separate family under the name Nimravidae (see Carnivora). The later forms, as well as some of the earlier ones, are more specialized as regards dentition than the modern Felidae, although in several other respects they exhibit more primitive features. The general type of dentition is feline, but in some instances more premolars are retained, as well as a small tubercular molar behind the lower carnassial. The characteristic feature is, however, the great development of the upper canines, which in the more specialized types reach far below the margin of the lower jaw, despite the development of a flange-like expansion of the extremity of the latter for their protection. In these extreme forms it is quite evident that the jaws could not be used in the ordinary manner; and it seems probable that in attacking prey the lower jaw was dropped to a vertical position, and the huge upper tusks used as stabbing instruments. The group is believed to be derived from a creodont allied to the Eocene Palaeonictis (see Creodonta).

Nimravus, of the American Oligocene, with two premolars and two molars in the lower jaw, and comparatively short upper canines, seems to be the least specialized type; next to which comes Hoplophoneus, another North American Oligocene genus, in which the tubercular lower molar is lost, and the upper canine is longer. It is noteworthy, however, that this genus retains the third trochanter to the femur, which is lost in Nimravus. Machaerodus, in the wider sense, includes the larger and more typical forms. In the Pliocene of France and Italy it is represented by M. megantereon, a species not larger than a leopard, and allied forms occur in the Pliocene of Greece, Hungary, Samos, Persia, India and China, as well as in the Middle Miocene of France and Germany. Far larger is the Pleistocene M. cultridens of the caverns of Europe, with serrated upper tusks several inches in length. From Europe and Asia the sabre-toothed tigers may be traced into North and thence into South America, the home of M. (Smilodon) neogaeus, the largest of the whole tribe, whose remains occur in the Brazilian caves and the silt of the Argentine pampas. This animal was as large as a tiger, with tusks projecting seven inches from the jaw and very complex carnassials; the feet were very short, with only four toes to the hind-pair, and the humerus has lost the foramen at the lower end. Very noteworthy is the occurrence of an imperfectly known specialized type — Eusmilus — in the Lower Oligocene of Europe and perhaps also North America. Unlike all other cats, it had only two pairs of lower incisors, and the large cheek-teeth were reduced to the carnassial and one premolar in advance of the same. (R. L.*)